1. Field of the Invention
The invention relates to an integrated circuit having an input circuit for receiving signals via an input terminal. In particular, the invention relates to an integrated circuit which can be put into a burn-in mode in order to pre-age the integrated circuit and which can be operated in a conventional manner after the burn-in operation.
2. Description of the Related Art
In conventional burn-in test systems for carrying out a burn-in method, a plurality of integrated modules are simultaneously subjected to a stress condition and are tested in the process. However, when initializing and driving the integrated modules which are to be tested in the burn-in method, only very slow signal ramps are possible since the tester channels provided for this purpose are simultaneously connected to a plurality or all of the integrated modules. In addition, the tester channels have a considerable amount of noise on account of their length, with the result that the input signals which are applied to the integrated modules are subjected to considerable signal disturbances. Specifically, in the case of a burn-in test system, the influence of the noise voltage on the tester channels may be ±100 mV. Noise in a clock signal or in a reference voltage signal may be interpreted as an additional clock pulse in the integrated modules, which may impair communication between the burn-in test system and the integrated modules.
In order to test integrated modules in a burn-in test system, the integrated modules are put into a test mode and are operated under extreme operating conditions, for example an increased ambient temperature, an increased supply voltage level and the like, as a result of which faults occur in susceptible integrated modules, with the result that said faults can be selected in a subsequent test operation. For the burn-in test operation, the integrated modules are usually operated in a burn-in mode which is assumed as a result of a particular signal sequence that is applied to the inputs of the integrated module.
An additionally interpreted clock pulse or a disturbance in a signal when applying the signal sequence for setting the burn-in mode in the integrated module may result in one of the signals in the particular signal sequence being detected twice during the test mode input sequence, with the result that the burn-in mode is not assumed under certain circumstances. In this case, the integrated module is not subjected to the burn-in test operation, with the result that the integrated module is not properly pre-aged.
The integrated modules have input circuits which can be used to receive externally applied signals. In order to be insensitive to input-side noise during the burn-in test operation, the input circuits of the integrated module must have sufficient hysteresis so that the noise does not lead to an additional clock edge, for example. However, input circuits having such high hysteresis are disadvantageous and are too slow for integrated modules having high transmission rates since the maximum operating frequency of the integrated modules is limited by the high hysteresis. By way of example, hysteresis of, at most, ±75 mV is permissible for an SDRAM module, in particular for a DDRII SDRAM 512 M T90, given an operating frequency of 590 MHz.